(1) Field of the Invention
The present invention relates to an imaging apparatus that processes luminance signals outputted form photodetectors so as to output image information, and an output device used in the imaging apparatus. More specifically, the present invention relates to a technique for suppressing deterioration of properties by reducing an adverse effect due to heat in a final output circuit.
(2) Description of the Related Art
In recent years, imaging devices such as home video cameras and digital still cameras has been increasingly popular.
These imaging devices typically employ a solid-state image sensor sequentially outputting signals from photodetectors that re aligned in two dimensional plane, using more than one vertical CCDs and at least one horizontal CCD.
Such a solid-state image sensor is detailed in “New Method of Driving CPD Solid-State Image Sensor” (Institute of Television Engineers of Japan Technical Report, Mar. 16, 2982) by Sone and 6 others from Electronics Research Laboratory of Matsushita Electronics Industry Corporation.
Further, a conventional art relating to heat dissipation for the solid-state image sensor is disclosed in Japanese Patent No. 2982353 (hereinafter referred to as Patent Document 1).
Patent Document 1 teaches that a constant current source unit in a final output circuit of output units is a source follower circuit and external to the solid-state image sensor, thereby reducing a heating value of the solid-state image sensor by half. However, the external constant current source unit still generates heat, and a total amount of heating value does not change. The technique of Patent Document 1 merely diversifies heat sources and does not refer to any other object and effect.
Generally, the photodetectors are positioned at a central part of the solid-state image sensor, and circuits that are not necessary for every photodetector are disposed on a peripheral area around the photodetectors.
Therefore, the output unit with a high heating value is disposed at a part of the peripheral area. When a thickness of a wafer of the solid-state image sensor is thinner than a certain degree, the heat generated at the output unit does not spread over an entire solid-state image sensor. Instead, a temperature rises only at a part of the photodetectors that are in a vicinity of the output unit.
It is a general tendency that an amount of dark current in the photodetectors increases as the temperature becomes higher.
Thus, the rise in temperature at a part of the photodetectors that are in a vicinity of the output unit increases the dark current in these photodetectors. This has an adverse effect on image qualities and makes a resulting image partially white.
Such a phenomenon can be observed when the wafer of the solid-state image sensor is as thin as around 500 μm, and becomes especially notable when the wafer is thinner than about 400 μm.
One of the prevailing approaches taken in order to avoid the above problem is to provide the output unit external to the solid-state image sensor having the photodetectors.
However, this approach has a problem that, when the output unit is external to the solid-state image sensor, output responsiveness decreases because floating capacitance in the wiring increases, and a S/N ratio deteriorates because of an increased amount of noise.